The present invention relates to a Faraday's rotator and an optical isolator for wavelength division-multiplex transmission systems, which makes use of the Faraday's rotator.
Optical communication systems and optical measuring instruments to which the wavelength division-multiplex transmission technique is applied has attracted special interest recently. The wavelength division-multiplex transmission technique permits fast and large capacity information interchange. It is generally observed that, in the optical communication system, light rays emitted from a light source are reflected by the end faces of various optical systems incorporated into the communication system and the reflected light rays are returned towards the light source. Accordingly, the optical communication system, in general, requires the use of an optical device for preventing the reflected light rays from returning back towards the light source. The optical isolator is used as such an optical device. The optical isolator used in the wavelength division-multiplex transmission system must have high light-extinction properties such that a plurality of transmitted light rays which fall within different wavelength ranges and are reflected and returned back towards a light source can effectively be cut off.
There have been proposed some attempts for improving optical isolators for wavelength division-multiplex transmission systems. For instance, T. Tamaki et al. proposes an optical isolator which can ensure a low insertion loss and always ensures a strong ability of extinguishing light rays transmitted through optical systems and reflected back towards the source thereof through reflection irrespective of the wavelengths of the light rays (see J. Appl. Phys., 1991 (Oct.), 70(8), p. 15). In addition, Japanese Patent Provisional Publication No. 4-242221 discloses an optical isolator which makes use of a Faraday's rotator whose Faraday's rotational angle is not affected by the wavelengths of transmitted light rays.
In the foregoing articles, the growth of crystal for the Faraday's rotators applied to optical isolators are performed according to the fluxing technique. However, the fluxing technique suffers from a problem of low processability because of its low reproducibility and low productivity. For this reason, this method does not, in fact, permit the production of optical isolators for wavelength division-multiplex transmission systems in high productivity and in a low production cost. In the optical isolator disclosed in Japanese Patent Provisional Publication No. 4-242221, the lattice constant of the crystal used in the Faraday's rotator is in a wide range extending from 12.426 to 12.484 .ANG.. Therefore, the crystal is prepared by depositing a crystalline layer on a samarium-gallium-garnet substrate (SGG substrate) having a composition represented by the compositional formula: Sm.sub.3 Ga.sub.5 O.sub.12 and having a lattice constant of 12.439 .ANG., through the liquid phase epitaxial crystal growth technique. The SGG substrate per se is quite expensive, the lattice constant of the substrate surface greatly varies depending on the production lots and accordingly, the crystal deposited thereon is easily cracked. This correspondingly results in an increase in the production cost.